Stand-by generators for supplying emergency power in case of a power failure are standard equipment in hospitals and other environments where maintenance of power under any circumstances is critical. These emergency sets are often rather sizeable diesel-powered machines and tend to be quite costly in spite of the fact that they are seldom used. Consequently, purchasers tend to specify a bare minimum capacity for the equipment which results in frequent though intermittent overloads when the equipment does go into action.
When overloaded, a diesel-electric generator goes into a lug mode which not only causes undesirable frequency and voltage fluctuations, but more seriously, causes mechanical stresses which are likely to do severe internal structural damage to the diesel engine in short order.
To prevent lugging, motor-generator sets are commonly provided with circuit breakers which trip when an overload occurs. This, however, is very undesirable in many instances; for example, in an operating room, it is much preferable for the lights to dim rather than to go out completely, even for a short period of time.